Terms

Introduction and Goals

Plants dominate the natural world and are the source of energy for the majority of other terrestrial organisms. Modern plants descended from an ancestral plant that lived in an aquatic environment. We will study the evolutionary history of the plant kingdom to better understand the selective forces that have shaped plants' development and led to the diversity of forms in existence today.

First we'll examine the similarities and differences between members of the kingdom Plantae and the red, brown, and green algae reviewed in the previous tutorial. Of the three classes of algae, green algae are the closest living relatives to modern land plants. Adaptations for the transition from an aquatic to a terrestrial habitat distinguish members of the plant kingdom, so these features will be discussed in detail. Plants have been evolving for at least 450 million years, and based on their major adaptive features, four major plant lineages (taxonomic groups) are currently recognized. This tutorial will introduce each of these groups. It will also describe the nonvascular plants, the most primitive of the plant lineages.

Throughout the tutorials discussing plant evolution and diversity, a good strategy would be to understand the major characteristics of each group, which characteristics are unique or are common to each group, and how these characteristics reflect adaptations to different environmental conditions. By the end of this tutorial you should have a working understanding of:

The origins of plant life and the major factors contributing to their evolution

Plant features that are adaptive to the terrestrial environment

The major plant lineages and features that are characteristic of each group

The distinguishing characteristics of nonvascular plants and their life cycles

Plants Share a Common Ancestor With Green Algae

Modern land plants have much in common with the group of green algae called charophytes, and charophytes are the closest relatives of the plant kingdom.

The phylogenetic tree on the right depicts the evolutionary relationships between charophytes and plants. It is thought that the first true plants were derived from a charophyte. This means that the ancestor species of all modern plants was actually a green algae living in an aquatic environment. There are many lines of evidence (e.g., chemical, structural, and genetic data) for the close evolutionary relationship between charophytes and plants. Chlorophytes (green algae) have the same photosynthetic pigments found in other plants.

Figure. 1 (Click image to enlarge)

Chlorophyll a is common to other photosynthetic organisms, but chlorophyll b is shared only by green algae and plants. Characters such as a cell wall composed primarily of cellulose, storage of carbon in the form of starch, and formation of a cell plate at cytokinesis are not limited to green algae and plants; however, these shared characters provide further evidence of their relatedness. Molecular evolutionary analyses of RNA and DNA sequence data from green algae and plants also clearly place these two groups closely together.

Despite the similarities between charophytes and plants, plants are classified in a separate kingdom (Plantae). Charophytes are highly adapted to an aquatic environment, and the features that distinguish members of the plant kingdom from charophytes are their adaptations to a terrestrial environment.

Geologic Time Scale for Plant Evolution

It is important to remember that ancestral plants had many more shared features with charophytes than those of modern plants. They would have been transitional between the green algae and the plants in existence today. The environment they occupied was most likely subject to periods of drying, such that they slowly evolved features that allowed them to exist in a terrestrial rather than aquatic environment. This theme of adaptation to environmental conditions, via natural selection, is very important in the study of plant evolution.

Fossil plant remains that clearly show features of true plants date back to 475 million years ago. The fossil record provides small windows into the past, and it aids scientists in approximating what life forms existed during a certain period in time. Because it can take millions of years for evolutionary changes to become established and because it is more rare than common for fossilization to occur, scientists estimate that the earliest plants began to evolve prior to the first evidence of plants in the fossil record. In fact, some evidence indicates that land plants appeared 700 million years ago.

The Transition From Aquatic to Terrestrial Environments

Although it is not certain when plants first arose, it appears that they did so during a time when the Earth's climate was changing. Likely, those areas where plants evolved was subject to periods of saturation and periods of drying, and characteristics that enabled some species to better survive during the dry periods evolved slowly. Adaptation to the drier conditions eventually enabled early plants to colonize the land. To fully appreciate the huge advantages that terrestrial migration had for plant development, it is necessary to understand the differences between aquatic and terrestrial environments with respect to requirements for plant growth.

In order for plants to photosynthesize and produce the proteins, lipids, and carbohydrates necessary for growth, they require light energy. Light energy received by organisms living beneath the water's surface is greatly reduced. The blue and especially red wavelengths of light that are absorbed by photosynthetic pigments do not penetrate deep beyond the surface of the water; therefore, photosynthetic organisms living in an aquatic environment do not receive the full amount of light energy radiated from the sun. Alternatively, photosynthetic organisms growing on land do not face this problem. Photons emitted from the sun can directly strike light-absorbing surfaces and the full range of useful wavelengths are available for photosynthesis. In the aquatic environment, many large algae compete for sunlight (similar to the competition for sunlight in modern forests). For early plants beginning to migrate to terrestrial environments, there was no competition for access to light.

It is important to remember the fundamental role that plants play within an ecosystem. Plants and other autotrophs are the basis for supporting heterotrophic life. Prior to colonization of the land by plants, there was little basis for support of animal life. As mentioned above, the vast majority of life existed in the ocean, including herbivores that depended on algae for food. A great advantage to the terrestrial migration of early plants was the lack of herbivores on land. Compared to life in the ocean, the terrestrial environment provided free access to sunlight and freedom from damage by larger organisms that could crush or eat the developing plants. However, the conditions on land were not completely hospitable for early plants.

Challenges to Terrestrial Occupation: Desiccation and Upright Growth

The major challenge for early plants first migrating onto land was the lack of water.

In an aquatic environment, desiccation is generally not a problem and there is no need for any protective covering to prevent water loss. Lacking any protection from the dry terrestrial environment, early plants likely became desiccated very quickly.

The ancestors of early plants were highly dependent on water, not only to maintain their moisture content but also for structural support. The buoyancy of water supports upright growth of giant marine seaweeds (e.g., kelp). Consider the seaweeds that are often found washed up on the beach. Although these algae are no longer alive, when held beneath the water their upright form is restored. In a terrestrial environment, the surrounding media is air rather than water. Air does not provide any support for upright growth. The transition to land required changes in structural features, and, as will be discussed later in this tutorial, adaptations for structural support are key features used in plant classification.

Adaptive Features of Plants

During the course of their evolution, plants have adapted to a land-based existence. Because modern plants occupy numerous, very specialized, ecological niches (e.g., deserts, rainforests, and even aquatic environments), there are many more specific adaptations than those that will be covered in this tutorial.

We will focus on those adaptations that have allowed plants to overcome the challenges of a dry environment and the lack of support for upright growth. These adaptive features include: cuticles, stomata, vascular tissue, gametangia, and seeds. As each of these adaptive features is discussed, keep in mind the transition of early plants from an aquatic to a terrestrial environment and how each feature could enhance the success of plants on land.

Waxy Cuticles and Stomata

A major adaptation to the dry terrestrial environment is the waxy cuticle. Cuticles, composed of wax, are found on the surface of all above-ground parts of the plant. Waxes are a class of lipids (discussed here under lipids) that, due to their chemical properties, are maintained as a solid, even at the highest temperatures found in extreme conditions (e.g., deserts). Like all lipids, waxes are hydrophobic and impermeable to water. Of course, plant roots are not covered by cuticle because they are the structures responsible for water uptake.

The waxy cuticle covering the surface of the plant shoot is an effective barrier to desiccation because it prevents loss of water to the air. Not surprisingly, desert plants have a much thicker cuticle layer than plants growing in wet environments.

Stomata (introduced here) are also an important adaptive feature to the terrestrial environment. Because the cuticle is impermeable, it is necessary for plants to have pores through which gasses can be exchanged with the environment. Remember, carbon dioxide is required for photosynthesis and oxygen is produced during this process. These gasses enter and exit the plant through the stomata.

Vascular Tissue

Vasculature describes a system of specialized cells found throughout the body of the plant. Vasculature has two functions. First, the specialized cells of vascular tissue allow transport of water and nutrients throughout the plant. This adaptation enables water, absorbed by the roots of the plant, to reach the stem and leaves, and the sugars from photosynthesis, produced in the shoots, to be transported to the roots. Plants with vasculature are less dependent on a very moist environment to maintain hydration throughout the plant.

The second function of vasculature is structural support. Cells of the vascular tissue have secondarily reinforced cell walls that make the tissue rigid. The vascular tissue that runs throughout the plant body, circulating water and nutrients, also forms a "skeleton" that strengthens the roots, shoots, and leaves. Vascular tissue enables plants to grow upright (some to very great heights), while maintaining moisture levels in all parts of the plant.

The evolution of vasculature was a major event in plant history. Plants with vascular tissue do not appear in the fossil record until approximately 400 million years ago, well after the origin of land plants. After this date there was an explosion of plant life, indicating that vascular tissue is a highly successful adaptation to life on land.

Gametangia

The transition from an aquatic to a terrestrial environment was also marked by various adaptations related to plant reproduction. In the charophyte ancestor of modern plants, processes such as gamete production, fertilization, and development of the embryo were highly dependent on the aquatic environment. Gametes were dispersed by water currents and were maintained in a hydrated state until fertilization occurred. The zygote and growing embryo developed free from the parent organism because there was no threat of desiccation. The move to land required protection from desiccation of gametes and embryos, as well as a new means of gamete and embryo dispersal.

The major adaptation of plants to the terrestrial environment (with respect to reproduction) was the production of gametes and the development of embryos within gametangia. The gametangium (-ium, singular; -ia, plural) can be male or female, and is the site of gamete production. The female gametangium produces egg cells and the male gametangium produces sperm. A protective chamber, formed by a single layer of sterile cells, prevents the gametes from drying out by reducing or eliminating their exposure to air. Egg cells are maintained in the female gametangium, but the sperm must leave the male gametangium for fertilization to occur. Some groups of modern plants have retained the primitive characteristic of flagellated sperm and still are dependent on water for dispersal of male gametes; however, the majority of modern plants do not have motile sperm and have developed nonwater-based methods of dispersal (e.g., wind and insect pollination).

In all plants, fertilization occurs within the female gametangium, where the zygote begins to develop into the embryo. Because all plants retain the developing embryo within the gametangium, they are referred to as embryophytes. Protection of the growing embryo is especially important in the terrestrial environment because the waxy cuticles, stomatas, and vascular tissue present in mature plants are not well developed in the embryonic plant.

Protection of the Developing Multicellular Embryo, and Seeds and Their Dispersal

Protection of the developing multicellular embryo varies among the different plant lineages. The most primitive group of plants retains the developing embryo through sexual maturity. The diploid embryo is completely dependent on the haploid gametophyte generation. This life cycle is typical of the nonvascular plants, and will be described in detail near the end of this tutorial. The more-derived plant lineages have further adapted to the terrestrial environment by producing specialized structures for protection and nutrition of the developing embryo. The embryo is enclosed in a seed, which is dispersed from the parent plant long before the embryo reaches maturity. In the derived plant lineages, the haploid gametophyte is greatly reduced because it no longer plays a dominant role in protecting the embryo; in these groups of plants, the haploid gametophyte has become completely dependent on the diploid generation. As you will learn in future tutorials discussing seed plants, seeds are a highly successful adaptation to the variable environmental conditions on land. Independent of the parent plant, the seed-enclosed embryo can withstand drying and temperature fluctuations, even the digestive tract of some animals, until conditions are suitable for germination and growth of the embryo to maturity.

The most recent adaptations to the terrestrial environment were the evolution of flowering plants and the production of fruit as a means for seed dispersal. Flowering plants produce their seeds within a fruit that provides a functional "packaging" around the seed(s). The fruit can be edible, such that the digested seeds are then deposited with the feces of the animal that consumed the fruit. Other fruits are suitable for transport on air currents, water currents, or on the fur of different animals. You will learn more about flowering seed plants (and their remarkable adaptations to life on land) in future tutorials.

The first evidence of seed plants in the fossil record occurs approximately 375 million years ago. Seed production enabled plants to reproduce more successfully because the embryos had a much better chance of surviving the dry terrestrial environment than did the embryos of more primitive plants that were still dependent on the parent plant body. Fruit production by flowering plants is a more specialized adaptation to life on land because it reflects not only the environment, but also the other life forms that exist there. Although the first flowering plants occur in the fossil record only 175 million years ago, the success of fruit production is marked by the huge radiation of flowering plants. Just think about the advantage to plants whose offspring could be widely dispersed and were protected (within the seed) until conditions were suitable for growth. Seed plants are so dominant in the world today that we have to remind ourselves that there are numerous plants in existence that do not produce seeds.

Plant Phylogeny: Lineages are Defined by Major Adaptive Features

The various adaptations to the terrestrial environment (e.g., waxy cuticles, stomata, vasculature, gametangia, seeds, and fruit) have evolved slowly during the 475 million-year history of plants. With these adaptations in mind, we will move on to a discussion of plant phylogeny and begin our review of the major characteristics of each of the plant lineages.
column}

The relationships indicated by the branching pattern of the phylogenetic tree shown on the right reflect the current character states and the evolutionary history of these groups of plants. The base of each of the plant lineages is defined by a significant adaptation to life on land (e.g., the development of vascular tissue, production of seeds, and flowering). As mentioned earlier, there are many more specialized adaptations to the terrestrial environment found among modern plants; however, these adaptations are found in only a small group of plants. The major adaptations that define each lineage are characteristic of hundreds and thousands of species of plants. The ancestral forms of these plants and those in which the adaptive features first evolved are no longer living, but their remains are present in the fossil record and provide a basis for dating the origin of each lineage. Continuous adaptations to an ever-changing environment over the history of plant life has led to the diversity of species currently in existence.

Figure 1. (Click image to enlarge)

Before we begin to discuss each of the plant lineages, it is important to understand the phylogenetic relationships among them. According to this figure, are nonvascular plants "older" than nonflowering seed plants? The correct way to interpret a phylogenetic tree is to read which groups are more closely related to one another, and which groups are more primitive or more highly diverged. Nonvascular plants are not "older" than nonflowering seed plants, but they possess a greater number of primitive character states than do nonflowering seed plants. Also, the origin of the nonflowering seed plant lineage occurred later in time than the origin of nonvascular plants, but this does not mean that currently living nonvascular plants are any older than currently living nonflowering seed plants. Flowering seed plants are the most derived lineage of plants.

Introduction to the Major Taxonomic Groups of Plants

Now that you have a working knowledge of the major adaptations present throughout the plant kingdom and understand the evolutionary relationships among them, you will be introduced to the four lineages: (1) nonvascular plants, (2) seedless vascular plants (3) nonflowering seed plants, and (4) flowering seed plants. Nonvascular plant characteristics will be examined in this tutorial in some detail. Upcoming tutorials will focus on the other groups. Always keep in mind how the adaptations found in each lineage of plants reflect the environmental conditions in which each lineage developed.

Nonvascular Plants

As the name of this group indicates, plants in this lineage do not have vascular tissue (or if present, it is very reduced). Because they lack substantial vasculature, plants in this lineage are generally small in size, lack significant structural support, grow close to the ground in moist areas, and lack significant water-conducting cells. Plants first evolved in environments that were transitional between the land and the sea, and although modern nonvascular plants are dependent on water to complete their life cycles, they are able to withstand long periods of desiccation. Nonvascular plants include mosses, liverworts, and hornworts.

Seedless Vascular Plants

Seedless vascular plants have a waxy cuticle, stomata, and well-developed vascular tissue. Their vasculature allows them to grow to larger sizes than the nonvascular plants, but they still generally occupy moist habitats. This lineage is more highly derived from the common ancestor of all plants than are the nonvascular plants, however, they do not produce seeds. Although the developing diploid embryo is dependent on the haploid gametophyte for survival (like mosses), the diploid sporophyte is more conspicuous and is the prominent generation of seedless vascular plants. Phylogenetically, seedless vascular plants are basal to the seed plants. The seedless vascular plants include species such as ferns and horsetails shown in the images below.

Fig 2 (Click image to enlarge)

Figure 3. (Click image to enlarge)

Vascular Plants With Seeds (Nonflowering Seed Plants)

Vascular plants with seeds belong to one of two groups: those with flowers, and those without flowers. Those without flowers are also known as gymnosperms, which means "naked seed" and refers to the lack of flowers and fruits in all members of this lineage (as compared to the more highly derived lineage with flowers). The ancestor of modern nonflowering seed plants evolved once plants were already well established on land. Adaptations of a well-developed vasculature and seeds indicate a strictly land-based existence. With the production of seeds, the gametophyte generation is highly reduced in this lineage and the sporophyte generation is most prominent. Nonflowering seed plants include conifers (e.g., pine, hemlock, spruce, and fir trees) and cycads. A cyclad is shown below on the left and a pine is shown on the right.

Figure 4 (Click image to enlarge)

Figure5 (Click image to enlarge)

Flowering Vascular Plants With Seeds

Flowering vascular plants are also known as angiosperms. Although this group of plants evolved more recently than nonvascular plants, seedless vascular plants and nonflowering seed plants, it includes the greatest number of species currently in existence. The great success of this group is due to their highly evolved and specialized methods for gamete dispersal (many species have insect or other animal pollinators) and seed dispersal (various types of fruits aid in the dispersal and successful germination of their seeds). The adaptations found in this taxonomic group are linked to the great diversity of animal life existing in the terrestrial environment. All of the agricultural crops we depend on for food, the fiberous plants we depend on for clothing, and the plants we grow to beautify our surroundings are from this lineage.

Figure. 6(Click image to enlarge)

Nonvascular Seedless Plants

We will discuss the various features of nonvascular plants, using mosses as an example. As we review the general characteristics of this group, remember, the liverworts and hornworts also belong to this group

Nonvascular Plants: Environment and Morphology

Of all the plant lineages, nonvascular plants are the most basal group. This means that throughout the millions of years of plant evolution, this group has retained more primitive characteristics than any of the other plant lineages. They are less derived than seedless vascular plants and seed plants, but this group of plants is highly successful in the environments they inhabit.

As one walks through a wooded area, they are highly likely to find mosses growing on rocks, rotting wood, trees, or on the ground. Nonvascular plants are generally small and do not extend much more than a few inches above the surface they are growing on. Their appearance can best be described as a "carpet of green." The plant body that is most obvious is actually the gametophyte generation, which is haploid.

Nonvascular plants typically grow in moist environments. Their lack of vascular tissue requires them to maintain close contact with water to prevent desiccation. They do not have true roots, true stems, or true leaves (which are distinguished by the organization of vascular tissue). Rhizoids are the root-like structures that function to anchor them to the surface they are growing on, however, they are not capable of water uptake. Water is absorbed throughout the "leafy" plant body of the gametophyte. They also require a moist environment for successful fertilization. They do not produce pollen grains, hence this group of plants has retained the primitive condition of a flagellated sperm. The male gametes are motile in water and must be released into a moist environment so that the sperm can swim to the female gametangium (where the egg cells are located).

Alternation of Generations: The Moss Life Cycle

Alternation of generations (discussed here) is an important concept in the study of plant evolution. During the life cycle of plants, generations alternate between the gametophyte (which produces gametes) and the sporophyte (which produces spores). As mentioned above, the most conspicuous (or prominent) form of all nonvascular plants is the haploid gametophyte generation.

To better understand alternation of generations in nonvascular plants, study this figure on the right. It depicts the life cycle of a moss and helps to distinguish the haploid and diploid stages.

Figure. 7(Click image to enlarge)

Note the prominent form of the moss, the gametophyte, which is haploid (1n). This is the plant body that is most often observed. In the figure there are separate male and female gametophytes; however, the gametophyte can also be bisexual (male and female gametangia are located on the same plant body). In step 1, the gametophyte is the generation that produces gametes; sperm are produced in the male gametangium, the antheridium (plural, antheridia), and eggs are produced in the female gametangium, the archegonium (plural, archegonia). In step 2, the motile sperm has reached the egg, which is retained in the archegonium and fertilization takes place. (Remember, water is required for fertilization because the flagellated sperm must swim to the egg.) In step 3, the diploid (2n) zygote undergoes mitosis and begins to develop into the embryo (also 2n). In step 4, the embryo matures into the sporophyte, the diploid (2n) plant body. The sporophyte is the small, brown, stalked structure that one sometimes sees held above the main body of the moss. In step 5, meiosis takes place in the sporangium of the mature sporophyte and haploid spores are produced. In steps 6 and 7, the haploid spores are dispersed and each spore undergoes mitotic cell division to create a haploid multicellular gametophyte. The prominent haploid gametophyte is then ready to produce gametes (back to step 1).

An important feature of the moss life cycle is that the developing embryo is retained on the gametophyte plant body. This is an adaptation to the terrestrial environment because the embryo is protected from desiccation throughout its development into the sporophyte. (Remember the description of plants as embryophytes?)

If one thinks about haploid and diploid stages in terms of the animal life cycle, it will be difficult to make sense of the plant life cycle. As noted above, the plant life cycle includes alternation of generations, with a multicellular haploid stage. Review again, step 5 of the life cycle (where meiosis takes place). Note that meiosis does not occur again when gametes are produced. Once you recognize these differences, you should begin to feel more comfortable thinking about the plant life cycle.

Summary

The diversity of plants existing today is the result of 450-700 million years of evolution and adaptation to the terrestrial environment. The common ancestor of all plants is thought to be very similar to species in the group of green algae known as the charophytes. Charophytes are similar to modern plants. Both have cellulosic cell walls, cell plates during cytokinesis, carbon storage in the form of starch, possession of chlorophyll b as an accessory pigment, and similar RNA and DNA sequences for particular genes. Charophytes are aquatic organisms, and it is highly likely that the earliest plants occupied transitional environments between the sea and the land. The transition to the terrestrial environment was advantageous for plants because there was direct access to sunlight and little to no herbivore activity. Early plants were ill-equipped for life out of the water, and desiccation was a major challenge to a land-based existence.

Adaptations to the terrestrial environment enabled generation after generation of plants to successfully exist out of the water. The waxy cuticle and stomata were effective in reducing water loss and preventing desiccation. Vascular tissue further reduced the problem of desiccation because it allowed transport of water and nutrients throughout the plant. Upright growth for improved access to sunlight was also an advantage conferred by vascular tissue because it also functions in internal support of the plant body. Protection of gametes and developing embryos was accomplished by evolution of the jacketed gametangia, and later, the evolution of seeds. More recently in plant history, adaptive features have been influenced by other organisms in the terrestrial environment; some plants produce specialized flower structures and fruits that attract insects and other animals that aid in pollination and seed dispersal.

All of the plants that are currently in existence are highly evolved, however, different taxonomic groups are defined based on the adaptive features that have or have not evolved. The most basal group is the nonvascular plants. They have retained many of the primitive characteristics that are also found in charophytes. Seedless vascular plants are more derived than nonvascular plants and are defined by their lack of seed production and presence of vascular tissue. The more derived lineages, nonflowering seed plants and flowering seed plants, both produce seeds, but only the flowering seed plants produce flowers and fruits.

The nonvascular plants (including mosses, liverworts and hornworts) are highly successful and can be found the world over. They are resistant to desiccation, but prefer a moist environment due to their lack of vascular tissue and motile gametes. Nonvascular plants, like other plants, are embryophytes, and their life cycles are based on alternation of generations. The prominent generation of nonvascular plants is the multicellular haploid gametophyte. The diploid sporophyte generation is completely dependent on the gametophyte for its survival. In the more derived plant lineages, the gametophyte is greatly reduced. Life cycles and the major characteristics of the seedless vascular plants, nonflowering seed plants, and flowering seed plants will be discussed in future tutorials for comparison with the more primitive nonvascular plants.